Statement about SPT

[Okiryu]

Hi, how do you interpret this:

"SPT tests shall be conducted continuously to a depth of 4 m, or 4 m below the lowest floor elevation, whichever is greater."

How can you do a "continous SPT"?. I usually see SPTs conducted at 1.5 meters intervals (US practice) or 1 meter intervals (in Japan). I am not sure how the above statement can be accomplished.

 

[Ron]

"Continuous" sampling and SPT values can be specified and done. Most good drillers know how to do this.

In general, the sampler is advanced for SPT values, then removed and the hole is cleaned only to the depth of the sampler, not to the next sampling interval as you've noted. The sampler is then advanced for the next SPT.

Yes, technically it is incorrect to call it "continuous" since each SPT will consume at least 18 inches of soil with only 12 inches counting for the SPT....but it's close.

 

[Okiryu]

Ron, as always thanks for your response. Is this procedure commonly done in the US? If I have clays, I would prefer to spend the money in taking an undisturbed sample instead do a "continuos" SPT, though.

 

[Ron]

Yes, it is often done when there is suspected significant stratification in the upper 10 to 20 feet.

I agree with you that taking an undisturbed sample for clayey soil is often more beneficial, both for checking soil lenses and for testing.

 

[oldestguy]

Recognizing it is a crude testing device, a firm I was with had all samplers 30 inches long (sampling length). If there was no caving problem, it was standard practice to drive the spoon 30 inches at a crack and down the former "hole" to 10 feet of depth, no clean out. That meant it took four steps to get to 10 feet. Only the second and third blows were added, but all 6" interval blows were recorded on the log. If conditions changed within the 30 inches, more than one jar was used. All jars were 4" high olive jars, so the sample could be compressed to get Qu values. Pocket pen also used. Maybe not ASTM, but we knew what were getting. The drawn log showed N values every 2.5 feet in that depth zone. It was common to do this sort of sampling to 20 feet for most building jobs. A problem with those spoons was that they bent quite easily, so a large stock was kept on hand. On rainy days the crew was busy in the shop at the press straightening them. But then again, you can't beat that old wash and chop method for getting info.

 

[Okiryu]

I talked with the drillers today and they told me that they did some "Continuous" SPT in the past. However, they have not done these in the last years. Anyway, good to know that this can be done if the client asks for this. Thanks for your responses !!

 

[fattdad]

I would interpret continuous SPT sampling as advancing the sampler the sampler's length using the hammer, reaming out the hole using the drilling equipment and then re-inserting the SPT sampler for the next interval for continuous sampling.

The method suggested by, "Oldestguy" is not appropraite and inconsistent with ASTM, so we would not allow that in highway work. I've seen it though and may be as old. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

OG again: Agree with fttdad, but if you recognize the complicating factors, it serves a purpose. One should look at the record of no claims, cheaper drilling cost, etc.and make a choice. Where cost is no object, sure clean out the hole first.

 

[Panars]

In my area of practice (Ohio USA), continuous SPT sampling is often done in landslide investigations. The intent is that you will be able to locate a thin weak layer that you might otherwise miss with SPT samples spaced 2.5 feet apart.

 

[BigH]

Well, an interesting take by previous replies. I have never run across "continuous" split spoon samplers (from which the SPT N value is obtained) - in Ontario (Golder and Geocon) we always did the sampling at 2.5 ft intervals to 15 ft, then at 5 foot intervals thereafter. In New Jersey, our driller would do three 24 inch drives to 6 ft, then go to 7.5 ft for the next test.

That's besides the point. I would suggest to the client that the thin-walled steel tube sampler is a more appropriate method of obtaining samples as well as giving a value (either by miniature lab vanes in the tip) or unconfined or triaxial testing for the undrained shear strengths as the OP is in clayey materials. Sometimes those who specify should leave the judgement to the geotechnical "expert".

When I was working in India, they would take an SPT at the surface, then a shelby at a depth of 3 m, then SPT at 6 m, etc. - really not good for what we were needing. I wanted to get a handle on the upper 6 m and 2 tests were not enough. As we were using 100 mm thin walled tubes, I would push a tube at, say, 1 m (1 to 1.45), then do an split spoon sampler (SPT) through the hole formed by the thin walled tube fomr 1.45 to 1.90. Drill to 2 m and do the same again. This way I got 6 thin walled samples and 6 disturbed samples (with SPT) in the first 7 m of the site. The bottom line is that we adjusted to the site conditions and didn't follow "standard" or "specified" protocol just to please when the protocols were not suitable.

 

[Okiryu]

BigH, if I understand it correctly, in India, your SPTs were at approx. 1m intervals? Also, I like your approach to take thin wall samples instead doing a "continuous" SPT....I will consider this for my next project, if the client asks for "continuous SPTs".... thanks !!

 

[emmgjld]

In Western Colorado, for most conditions, I try to use the Shelby Tube & then drive an SPT through the Shelby hole, then auger out to clean. Using a 24" Spoon & about 12"+ Shelby push, I end up with 3'+ of sample in a 5' auger run. I have modified the sequence for a near continuous sample run.
In hard or gravelly soils, I have driven a 24" long 'California' Sampler (w/liners) & then a 24" SPT in the existing hole, then auger out to clean.

Beware of trying to run a Shelby immediately beneath a drive, unless you are aware of what can be happening in the soil in front of the drive head. I have had a few surprises.

 

[Okiryu]

I have heard about the California Sampler. My understanding that it is used in granular-type soils. Unfortunately, we do not have a similar sampler here, so we are limited to the SPT sampler for granular soils. BTW, when we want to take undisturbed samples, unless we are very sure about the depth and type of soil, we take the undisturbed samples in another borehole. Costly, but that is the common practice here.

 

[mikeofBelAir]

I might be showing my age, but, along with oldestguy, I have done continuous SS sampling--in the past. Often it was done using a tripod rig and a simple cathead hoist and a 2 piece ( non-safety) hammer on remote sites. The entire hole was advanced by driving one SS after another until the bottom of the hole was reached--often 10'. We used 24" samplers and drove them 18", recording the blows for each 6". On some occasions we used a 2 1\2" spoon to clean out the hole in order to reduce friction above the new sample. Not saying it was "correct", but it was a common practice and gave useful information--especially in the days before small ATV rigs were available. All this stuff was hand-carried to the location of each hole.

 

[fattdad]

this has nothing to do with age or equipment. The ASTM for performing the SPT is not new. Make a hole and advance the sampler from the bottom of the hole. It doesn't matter if you are hand-carrying the tripod or not. It doesn't matter if you are doing the SPT every 10 ft or every 18 inches. Heck, in Panama, we'd advance 20 ft holes with hand auger and just use the tripod for the donut. Each SPT though began in a hole drilled to the sample depth.

p.s., I got gray hair too!

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

Thanks again for your comments.

f-d, 20 ft deep hand auger should be tough, what tools did you use?